1. Field of the Invention
The present invention relates to a control device of a lock-up clutch for controlling an engagement state of the lock-up clutch with which a torque converter is provided, and particularly, the present invention relates to a control device capable of controlling rise in facing temperature due to slip control for the lock-up clutch.
2. Description of the Related Art
In an automatic transmission mounted on a vehicle, a torque converter for amplifying torque has been utilized. A lock-up clutch for directly connecting an impeller to a turbine is provided in most of the torque converters. By providing the lock-up clutch to the torque converter, power transmission efficiency is heightened and fuel economy is improved. However, in order to improve the fuel economy further, it is desired that an engagement region of the lock-up clutch is broadened. On the other hand, since variation in engine torque is directly transmitted to a drive system by means of engagement of the lock-up clutch, this causes an increase in noise or vibration.
Therefore, technique to inhibit transmission of the variation in the engine torque to the drive system by slipping the lock-up clutch slightly has been utilized. However, in the case where the lock-up clutch is continuously slipped, a surface of a friction member (facing member) becomes high temperature. This causes a mirrored process, abrasion, carbonization or the like of the friction member, and decrease of frictional force thereby occurs. Thus, there is a fear that a sufficient lock-up clutch capacity against the engine torque cannot be obtained. Further, when the surface of the friction member becomes high temperature, heat generation due to a work loss is increased by enlarging a slippage amount of the torque converter in addition to heat generation of the friction member. This causes hydraulic oil in the torque converter to become high temperature, thereby leading to acceleration of deterioration of a resin component or a rubber component. Further, when the rise in temperature in the torque converter becomes marked, there is a possibility that any hydraulic oil path in the torque converter is damaged in the worst-case scenario. In the event that such a situation occurs, a function to transmit the engine torque is markedly lowered.
In order to avoid such a phenomenon, Japanese Patent No. 3476718 discloses technique in which facing temperature of a lock-up clutch is estimated and temperature in a torque converter is lowered in the case where the facing temperature is estimated as high temperature. More specifically, in the case where the facing temperature of the lock-up clutch becomes high, by selecting either complete engagement of the lock-up clutch (slip ratio=zero) or complete release of engagement of the lock-up clutch, slip control for the lock-up clutch is suspended to lower the facing temperature and the internal temperature of the torque converter. However, there has been a problem as follows. In the case where the lock-up clutch completely engages, vibration and noise in driving force transmission is deteriorated. On the other hand, in the case where engagement of the lock-up clutch is completely released, fuel economy is deteriorated.
Further, U.S. Pat. No. 6,959,239 discloses technique as follows. Namely, internal temperature is estimated from a work loss of a torque converter (difference of workloads inputted into and outputted from the torque converter). In the case where the estimated internal temperature is high temperature, a slippage amount of the lock-up clutch is reduced to reduce the work loss, whereby oil temperature in the torque converter is lowered. However, facing temperature of the lock-up clutch depends not only upon the internal temperature of the torque converter but also upon other factors. Thus, in the case where the slippage amount of the lock-up clutch is controlled on the basis of the internal temperature estimated from the work loss of the torque converter, there is a fear that delay in timing to reduce the slippage amount occurs. Therefore, there has been a problem that the facing temperature cannot be controlled to become assumed temperature with high accuracy.